1. Field of the Invention
The present invention relates a method and a system for monitoring sedation, paralysis and neural integrity as is required in surgical and intensive care environments. In particular the present invention relates to a compact and affordable method and system for monitoring these parameters.
2. Field of the Invention
In the course of their practice, Anesthesiologist must monitor the effects of the agents they administer to their patients, and the effects of various surgical manipulations, to ensure the well being of their patients. Three important categories which require diligent monitoring from the anesthesiologist are as follows:
Sedation
Sedatives are administered to render a patient unconscious, prevent stress to the Central Nervous System, and eliminate pain. Care must be taken to avoid over-sedating, which wastes expensive anesthetic, delays patient recovery, and may contribute to other long-term effects including coma. Under-sedating is also a serious problem, which may cause patients to awaken during the procedure. Continual assessment of the correct level of sedation is therefore necessary. Anesthesiologist normally rely on clinical signs such as heart rate or blood pressure responses to surgical stimuli to ensure the adequacy of sedation, but these signs are not present in certain surgical procedures, such as heart bypass surgery, leaving the anesthesiologist without any measure of sedation. A patient""s electroencephalogram (EEG) may provide additional information, but the completed interpretation of raw EEG requires expert training. Parameters derived from the EEG such as Median Frequency or Bispectral Index may help in the interpretation, but these have not been found to be totally reliable in all cases. Derived parameters may be susceptible to artifact or noise; have varying responses with different agents; and provide no objective feedback to the clinicians when the data they are presenting is erroneous.
Mid-latency Auditory Potentials (MLAEP) have been proposed as a method to determine sedation which overcomes the difficulties seen with raw EEG and derived EEG parameters. MLAEP is obtained by recording the EEG while auditory tones are issued to the patient via headphones, and applying signal processing techniques to remove all but those signals which are correlated to the tone, so that over time, a waveform is generated. MLAEP has been shown to give a graded response to level of sedation, insensitive to different sedative agents. Artifact removal is proportional to the time average, so that arbitrarily clean signals may be obtained. The generated waveform may be inspected for accuracy, giving immediate confirmation that the data is accurate.
Neuromuscular Blockade
Neuromuscular blockage agents are given to induce temporary paralysis, allowing surgical manipulation without patient movement, either voluntary or involuntary. It is important that the paralysis is maintained throughout surgery, to prevent movement which might interfere with the surgeon. As each patient responds differently to the agents, monitoring of the effect is necessary to establish when a patient may be safely extubated.
A subjective indication of paralysis may be obtained by using a nerve stimulatorxe2x80x94a nerve such as the ulnar nerve is stimulated at the wrist, and the thumb response is observed. However, this method is not easily quantifiable, and the documentation must be done manually. An improved variant of this technique utilizes a method of measuring the thumb response directly, and performing calculations on this response to determine paralysis. Parameters such as Train-of-Four Percentage (TOF %) measure the ratio of the 4th to the 1st response, and Post-tetanic Count (PTC), count the number of responses which can be induced following tetanus. These parameters are objective, convenient to obtain, and may be documented automatically.
Neural Integrity
Certain surgical procedures such as spinal surgery may compromise the integrity of motor and sensory nerve pathways. Any surgery on or near the spine may damage the spinal cord, and any damage must be detected as soon as possible to prevent permanent injury. A standard procedure to attempt to detect any damage is to partially waken the patient, and instruct him/her to xe2x80x9cwiggle your toesxe2x80x9d upon command. This technique is slow, delaying the surgery and only gives sporadic, subjective measures of pathway integrity.
A superior method which gives continuous, objective measures is to use Somatosensory Evoked Potentials (SEP). Any electric stimulus is applied to a sensory nerve, such as the Radial nerve, and a response is observed in the patients EEG. Signal processing techniques similar to the MLAEP are performed to obtain a waveform of brain patterns correlated to the stimulus. Changes in the shape or latency (time delay since stimulus) give a graded indication of integrity. The brain response to a single stimulus from 2 different locations given an indication of propagation delay, providing an early indication of any injury.
Despite the availability of devices to facilitate continuous, objective monitoring as described above, such monitoring is not performed in every institution, or in every surgical case, because of the following problems:
Expense of the devices. Currently, 2 or 3 different devices are required to provide the full range of monitoring required. These devices are expensive, some costing $20,000 to $100,000. Thus a hospital may purchase a few devices for an entire surgical floor to share among many operating rooms, limiting their widespread use.
Inconvenience of use. These devices each require their own application to the patient, with associated sensors. The task of attaching the different sensors and probes may take 30 minutes or more. Each has its own interface to the clinician, and must be positioned in the crowded workspace of the Operating Theatre. Many cables must be run from the patient to the anesthesia station, crowding the workspace, increasing electrical noise, and risking accidental disconnection.
Complexity of use. These devices are usually adapted from the related diagnostic applications, and are poorly adapted to the monitoring application. In particular, the user interfaces are not optimized for ease of use in the surgical setting. The anesthesiologist is required to learn several different interfaces. Therefore it would be desirable to provide a method and system for monitoring these parameters which overcomes the aforementioned drawbacks of the prior art proposals.
The present invention provides a method and a system for integrating all of these anesthesia monitoring functions into a single module, which has the advantages of small size, low cost, ease-of-use, minimal cabling, and integration into a single user-interface at the anesthesiologist""s monitor in which a small stimulator unit which is designed to be mounted near the patient. A single cable connects the stimulator unit to the Patient Monitor. This cable both provides power to the stimulator unit, as well as 2-way data communication between the stimulator unit and the Patient Monitor. The Patient Monitor provides the user interface for the present invention, as well as other parameters such as Heart Rate, Blood Pressures, etc.